Raised in January 1942, amidst concerns of a Japanese invasion of Australia, the brigade was tasked with defending the eastern New South Wales coast around Botany Bay and Cronulla.

The 31st Brigade was formed shortly after Japan's entry into the war, when the Australian government was concerned about the possibility of a Japanese invasion.

The brigade opened its headquarters at Burwood, New South Wales, in January 1942, although it moved to Blakehurst the following month.

[4] Under the command of the 2nd Division, the brigade moved to Narellan to complete training, remaining there until June 1942, when they were rotated back to Blakehurst.

At this time, the forward defensive positions along the beach had been taken over by the 18th Garrison Battalion, so the 31st Brigade's troops continued to carry out training, while tasked with mobile defence in the event of an invasion.